The user's preference usually reflects the displayed skin, however this isn't
true if the `useskin` URL query is set, or MobileFrontend is being used.
This fixes gadgets being displayed on the wrong skin when using `useskin`,
and allows mobile-specific gadgets (using `skins=minerva`).
Testing if the gadget is allowed in the current skin is split out from
`isAllowed` to `isSkinSupported` to enable a future patch showing gadgets
on preferences regardless of if they are allowed on the current skin.
Test coverage is added for both functions.
Also fixed another test which wasn't working, presumably because the placeholder
user didn't have the "read" right, so the section wasn't being kept.
Bug: T199478
Change-Id: I21febe92d54d6d0b89925f902581cc2739d824fb
